1. Field of the Invention
The present invention relates to a directional coupler, an antenna device, and a transmitting-receiving device which are useful for a radar or the like with which the distance to and the relative velocity of a detection object are measured by transmission-reception of an electromagnetic wave, for example, in the millimeter wave band.
2. Description of the Related Art
In recent years, a so called xe2x80x9cmillimeter wave radar for car-mountingxe2x80x9d has been developed, of which the purpose lies in that the distance to and the relative velocity of a vehicle running ahead or behind are measured in a vehicle running on a road and so forth. In general, the transmitting-receiving device of the millimeter wave radar of the above type includes a module comprising a millimeter wave oscillator, a circulator, a directional coupler, a mixer, an antenna, and so forth which are integrated together, and is attached to the front or rear of the vehicle.
For example, with the module of this type, the relative distance and the relative velocity of a vehicle running ahead are measured at a vehicle running behind, by transmission-reception of a millimeter wave according to the FM-CW system or the like. The transmitting-receiving device and the antenna of the module are attached to the front of the vehicle, and a signal processing device is disposed in an optional location of the vehicle. In the signal processing section of the signal processing device, the distance to and the relative velocity of the vehicle running ahead are extracted as numerical information. In the control-alarm section, based on the velocity of the vehicle running behind and the distance between the vehicles, an alarm is given, for example, when predetermined conditions are satisfied, or when the relative velocity for the vehicle running ahead exceeds a predetermined threshold.
In the millimetric radar of the above type, the directivity of the antenna is fixed. Therefore, there may occur the case that the desired detection or measurement can not be performed depending on conditions, as described below. More particularly, for example, if vehicles run in plural traffic lanes, it can not be determined immediately whether a vehicle running ahead is present in the same lane where the vehicle is running behind, only by receiving an electromagnetic wave reflected from the vehicle running ahead. More particularly, when an electromagnetic wave is sent as a radiation beam from the vehicle running behind, a reflected wave from the vehicle running ahead, and moreover, a reflected wave from a vehicle running in the opposite lane may be received. The relative velocity determined based on the reflected wave from the vehicle running in the opposite lane is unduly high. As a result, inconveniently, an error alarm is given. Further, if vehicles are running on a curved road, a vehicle running ahead is out of the detection range of the radiation beam and can not be detected, by sending forward an electromagnetic wave as a radiation beam from the vehicle running behind, Further, if vehicles are running on a hilly road, a vehicle running ahead in the lane where the vehicle is running behind is out of the detection range of the radiation beam, and can not be detected.
Accordingly, it is speculated that the above-described problems can be dissipated by varying the direction of the radiation beam.
For example, in the case that vehicles run in several traffic lanes, two detection objects adjacent to each other in the forward angular directions can be separately detected by changing the radiation beam, operational processing, and comparing the measurement results in the respective beam directions. If the vehicles are running on a curved road, the curve of the road is decided based on the steering operation (steering by a steering wheel) or by analyzing the image information obtained with a camera photographing the forward view, and the radiation beam is directed to the direction in dependence on the decision, so that the vehicle running ahead can be detected. Further, if the vehicle is running on a hilly road, the undulation of the road is decided by analysis of image information obtained with a camera photographing the forward view. The radiation beam is directed upwardly in dependence on the decision, so that the vehicle running ahead can be detected.
However, referring to the method of changing the directivity of an electromagnetic wave in the conventional transmitting-receiving device operative in the microwave band or millimeter wave band, the whole of a casing containing the transmitting-receiving device including the antenna is rotated only with a motor or the like to change (tilt) the direction of the radiation beam. Accordingly, the whole of the device is large in size, and it is difficult to scan with the radiation beam with the direction of the radiation beam changed at a high speed.
Conventionally, by another method, beam-scan antennas for switching plural antennas to scan with a beam are employed. However, by the beam scan antenna method, it is necessary to provide as many antennas as beams. Accordingly, if the beam scan antenna is used in the transmitting-receiving device, there is caused the problem that the whole size of the device is large. Further, since as many antennas as beams are used, it is needed to arrange the respective antennas in consideration of their scan ranges. Thus, the arrangement of the antennas is difficult. Further, in order to switch the plural antennas for inputting or outputting, electronic switches such as diodes or the like are used. The loss at the switching is too large to be neglected in the millimeter wave band. Further, it is needed to switch on-off the beams from the plural antennas, and therefore, it is necessary to provide as many electronic switches as antennas. The electronic switch such as a diode or the like is expensive. Thus, there is the problem that the beam scan antenna using many electronic switches costs a great deal.
In recent years, investigation on three dimensional beam scanning by which upper, lower, right, and left sections are scanned has been made. If a method of moving the whole casing of the transmitting-receiving device only by means of a motor or the like is employed, there is caused the problem that the whole structure is further enlarged, and the scanning at high speed is difficult.
Further, for three dimensional beam scanning by means of a multi-beam antenna, it is needed to arrange antennas in the upper, lower, right, and left sections. Thus, there is caused the problem that the whole structure is large in size, and the connection, switching, and arrangement of the respective antennas is very difficult.
Accordingly, it is an object of the present invention to solve the above problems and to provide a directional coupler with which switching on-off can be performed by changing the relative positions of two transmission lines, an antenna device, and a transmitting-receiving device which can be easily miniaturized and of which the directivity can be switched at a high speed, respectively, due to the directional coupler.
According to a first aspect of the present invention, there is provided a directional coupler including a first transmission line and a second transmission line which are partially opposite to each other, the opposite portions of the first transmission line and the second transmission line being relatively shiftable in parallel and operative to be shifted from their opposite state to their non-opposite state.
With the above structure, the coupling portion of the directional coupler can be used as a switch.
In the directional coupler in accordance with the present invention, either of the first transmission line and the second transmission line may comprise plural transmission lines.
Accordingly, the plural transmission lines can be switched.
According to a second aspect of the present invention, there is provided a directional coupler including a first transmission line and a second transmission line which are partially opposite to each other, the opposite portions of the first transmission line and the second transmission line being relatively shiftable in parallel, the first transmission line being capable of being connected by the parallel shift of the first transmission line, to plural third transmission lines individually which are on the opposite side to the opposite portions of the first transmission line and the second transmission line.
With the above structure, the plural lines can be switched.
Preferably, there is provided an antenna device including the directional coupler in accordance with the present invention, a primary radiator connected to the first transmission line, and a terminal resistor connected to one end of the second transmission line.
With the above structure, the transmission and reception through the antenna can be switched.
Also preferably, there is provided an antenna device containing the directional coupler in accordance with the present invention, plural primary radiators connected to the first transmission line and a terminal resistor connected to one end of the second transmission line.
With the above structure, beam scanning with plural beams is enabled.
Preferably, in the antenna device, the first transmission line consists of plural transmission lines, a primary radiator is connected to at least one of the plural first transmission lines, one of the plural first transmission lines, not connected to the primary radiator, functions as a measurement terminal.
With the above structure, the output characteristics of the antenna in the coupling state caused by the directional coupler can be measured.
Preferably, in the antenna device, the terminal resistor is removable, and one end of the second transmission line having the terminal resistor connected thereto is used as a measurement terminal.
With the above structure, the characteristics of the antenna device prior to the coupling by use of the directional coupler can be measured.
Preferably, there is provided a transmitting-receiving device including the antenna device in accordance with the present invention.